Petroleum products have so-called temperature dependence of viscosity wherein a large variation in viscosity is exhibited with variation in temperature. For example, for lubricating oil compositions used for automobiles or the like, it is preferable that the temperature dependence of viscosity is small. Therefore, in order to decrease the temperature dependence of viscosity, a certain kind of polymer which is soluble in a lubricating oil base is used in lubricating oil as a viscosity modifier.
Ethylene/α-olefin copolymers are widely used as viscosity modifiers for lubricating oils, and various improvements have been made in order to further improve the balance of performances of the lubricating oils (for example, see patent Document 1).
In recent years, in view of the reduction of petroleum resources and environmental problems such as global warming, an improvement of fuel efficiency of automobiles which is aimed at reducing CO2 emissions is required. A significant effect with low cost can be expected from lowering fuel consumption by lubricating oils, as compared to physical modification of the lubricated machinery. Therefore, the requirement for improving fuel efficiency by lubricating oils is growing.
The power loss in an engine or transmission is divided into friction loss at a sliding part and agitation loss due to the viscosity of lubricating oil. In particular, reduction of viscosity resistance is one measure of lowering fuel consumption by engine oils. In recent years, fuel consumption is tested based on performance at low temperatures as well as that at high temperatures, and thus reduction of viscosity resistance in a wide temperature range from low temperature to high temperature is desirable for improving fuel efficiency.
Lowering viscosity is effective for the reduction of viscosity resistance of an engine oil. In particular, at a low temperature, reducing viscosity is effective for the reduction of both friction loss and agitation loss. However, this does not mean that the viscosity should be simply lowered, because abrasion is caused at a sliding part at a high temperature. In other words, it is desired that the viscosity is lowered as much as possible in order to reduce agitation loss at a non-sliding part while a certain minimum required viscosity is ensured to avoid abrasion at a sliding part.
In order to reduce low-temperature viscosity, it is known to use a polymer as described in Patent Document 1, wherein the polymer dissolves in base oil and provides excellent thickening properties at a high temperature, while the solubility of the polymer in oil is decreased at a low temperature, and thereby its effective volume (flow volume) and impact on viscosity are decreased.
Also, a viscosity modifier for lubricating oils comprising an ethylene/α-olefin copolymer containing a structural unit derived from ethylene and a structural unit derived from two or more kinds of α-olefins is known (for example, see Patent Document 2).
Regarding the viscosity modifiers described in Patent Documents 1 and 2, an improvement of fuel efficiency at low temperature is sufficient because the low-temperature viscosity of a lubricating oil composition containing said modifier is decreased. However, there is room for improvement regarding fuel efficiency at high temperatures.